I would suggest "Thermore" for your placemats, that's what I and most of the quilters I know use for tablerunner's and placemats, it is quite thin, so your glasses don't tip over if they get set on the edge.
Here's a little information about it, not sure what brand I've used but you get the idea here anyway.
http://www.connectingthreads.com/too...g_TD20825.html
There is also a fusible batting that you can use too,
http://www.connectingthreads.com/too...usCtnBtng.html